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Fair Value Measurements (Tables) | 12 Months Ended
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Dec. 31, 2012
Fair Value Measurements |
Schedule of assets and liabilities measured on a recurring basis, at fair value | The major categories of assets and liabilities measured on a recurring basis, at fair value, as of December 31, 2012 and 2011 are as follows (in millions):
|  | December 31, 2012 |
--- | --- | --- | ---
|  | Level 1 |  | Level 2 |  | Level 3 |  | Total |
Treasury bills |  | $ | 278.7 |  | $ | — |  | $ | — |  | $ | 278.7 |
Government agency securities |  |  | — |  |  | 123.0 |  |  | — |  |  | 123.0 |
Total |  | $ | 278.7 |  | $ | 123.0 |  | $ | — |  | $ | 401.7 |
|  | December 31, 2011 |
--- | --- | --- | ---
|  | Level 1 |  | Level 2 |  | Level 3 |  | Total |
Treasury bills |  | $ | 90.2 |  | $ | — |  | $ | — |  | $ | 90.2 |
FDIC guaranteed corporate debt |  |  | — |  |  | 114.8 |  |  | — |  |  | 114.8 |
Government agency securities |  |  | — |  |  | 169.8 |  |  | — |  |  | 169.8 |
Money market instruments |  |  | — |  |  | 0.2 |  |  | — |  |  | 0.2 |
Total |  | $ | 90.2 |  | $ | 284.8 |  | $ | — |  | $ | 375.0 |
The classification in the fair value table as of December 31, 2011 has been revised to conform to current period classifications due to an immaterial error related to previously disclosed fair value hierarchy tables.

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- Definition Tabular disclosure of assets and liabilities, including [financial] instruments measured at fair value that are classified in stockholders' equity, if any, that are measured at fair value on a recurring basis. The disclosures contemplated herein include the fair value measurements at the reporting date by the level within the fair value hierarchy in which the fair value measurements in their entirety fall, segregating fair value measurements using quoted prices in active markets for identical assets (Level 1), significant other observable inputs (Level 2), and significant unobservable inputs (Level 3). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19190-110258 + Details
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